Separator Design and Thermal Study of Ionic Liquid Electrolyte Lithium-ion Cells

Author: Candice Francis

Francis, Candice, 2018 Separator Design and Thermal Study of Ionic Liquid Electrolyte Lithium-ion Cells, Flinders University, College of Science and Engineering

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Safety concerns surrounding the flammability of lithium-ion cells under abuse conditions inspired this thesis. It is widely accepted that the reactivity and volatility of conventional electrolytes are a significant contributing factor, hence the need for safer electrolyte alternatives. One such electrolyte is based on ionic liquids.

Ionic liquids are comprised entirely of ions and can be liquid at room temperature. Ionic liquids have negligible vapour pressure and therefore very low flammability, which is advantageous for lithium-ion batteries. However, compared to conventional electrolytes, ionic liquid electrolytes have poor wetting properties, due to their hydrophobic / hydrophilic characteristics, which can negatively affect the performance of a lithium-ion cell. This thesis aims to investigate the thermal characteristics of ionic liquid electrolytes and address the wetting issues through the development of a novel separator.

It was hypothesised that the surface of a separator could be modified to improve wettability with an ionic liquid electrolyte. The novel separator was developed using electrospinning to modify the surface of a support membrane. The physical and electrochemical properties of the separator were investigated. The support membrane provided mechanical integrity and thermal stability to the separator, and the electrospun layers successfully increased separator wetting and electrode-electrolyte interphase stability. However, cycling of lithium-ion cells containing the novel separator and ionic liquid electrolyte were still not comparable to conventional electrolyte cells.

It was further hypothesised that a lithium-ion cell containing ionic liquid electrolyte and the enhanced separator would display superior thermal stability. Using Differential Scanning Calorimetry and Thermalgravimetric Analysis, the exothermic decomposition characteristics of the ionic liquid electrolyte were evaluated with a number of commercial electrode materials. It was found that the total heat generation during decomposition of these electrodes was not necessarily lower with the ionic liquid electrolyte than with conventional electrolytes. However, with most electrodes, the ionic liquid electrolyte was found to delay the onset of exothermic decomposition, compared to conventional electrolytes.

Properties extracted from the thermal stability investigation were used to implement a thermal model for a lithium-ion cell with an ionic liquid electrolyte. A generic lumped thermal model was used, containing Arrhenius equations to describe internal heat sources. This model has been validated in the literature for conventional electrolyte lithium-ion batteries, however, to this author’s knowledge, this is the first time the thermal decomposition of ionic liquid electrolyte in a lithium-ion cell has been modelled. Oven test simulation of the ionic liquid electrolyte lithium-ion cell revealed that the cell did not enter thermal runaway when exposed to 350 °C for 60 minutes. Whereas, simulation of an equivalent conventional electrolyte lithium-ion cell entered thermal runaway when exposed to 200 °C for approximately two minutes.

The investigations conducted in this thesis partially support the hypothesis that a lithium-ion cell containing ionic liquid electrolyte with the enhanced separator has greater thermal stability. In the event of thermal runaway, an ionic liquid electrolyte cell would release a larger amount of energy that could affect the safety of surrounding cells in a battery module. The delayed thermal runaway onset temperature provided by ionic liquid electrolytes, however, could give the battery management and cooling systems a greater opportunity to prevent a cell entering thermal runaway and resulting cell fires.

Keywords: Lithium-ion Battery, Separator, Ionic Liquid Electrolyte, Thermal Stability

Subject: Engineering thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Professor Karl Sammut